The objectives of the present proposal are to characterize the mechanisms of hormonal regulation of two key transporters essential for the maintenance of hepatic bile secretory function. The research proposed builds on our novel and exciting finding that in the postpartum period, the anterior pituitary hormone prolactin (PRL) increases the activity of both the Na+/taurocholate (TC) cotransporter (ntcp) in the basolateral domain, and the ATP-dependent TC transporter, recently identified as the sister of P-glycoprotein (spgp) in the canalicular domain of the hepatocyte. The PRL-induced increase in ntcp transcription is mediated by the long form of the PRL receptor and is transduced by the Jak2-Stat5 signal transduction pathway and two Stat5 recognition sequences (GLEs) in the ntcp promoter. Aim 1 will determine the role of other Stat proteins (1, 3, 5a, 5b) and other members of this hormone family (i.e., growth hormone and placental lactogen) in regulating ntcp and spgp expression in HepG2 cell culture and/or in vivo in lactating dams. Aim 2 will identify the mechanism(s) by which estradiol inhibits the PRL-mediated increase in ntcp expression, and determine if spgp expression is similarly regulated. Specifically, we will test the hypotheses that estradiol acts via the estrogen receptor and 1) binds the half-estrogen response elements in the ntcp promoter to prevent PRL-mediated signal transduction, or 2) competes for a limited pool of nuclear coactivators, e.g., CBP/P300, which are essential for initiation of transcription. Aim 3 will determine if PRL secretion in the postpartum period or infusion or PRL or growth hormone increase expression of ntcp and spgp mRNA and protein by increasing gene transcription. Approaches used to achieve these goals include transfection studies in a HepG2 cell culture model system, studies in vivo in nonpregnant control and postpartum rats, in ovariectomized rats treated with ovine PRL and/or estradiol and hyphophysectomized rats treated with growth hormone. PRL is the only physiological mechanism identified which increases expression of bile acid transporters. Characterization of the mechanism of these PRL actions presents the opportunity for development of selective therapeutic interventions for the treatment of human cholestatic liver disease, such as that induced by estrogens and in prematurity.